In a wiped or turbulent thin-film processing apparatus, particularly with vertically axised evaporators, it is particularly difficult to obtain good thin-film agitation on the interior wall surface as the amount of fluid processed through the apparatus from the feed inlet to the product outlet ends decreases in the thin film by evaporation. There have been attempts to overcome this difficulty and to control the flow and/or residence time of the fluid material in both horizontally and vertically axised thin-film evaporators. With liquid materials, for example, of low viscosity, it is often desirable to obtain a longer flow path or longer residence time, while for materials of higher viscosity, typically a shorter flow path or shorter residence time is required. Control of flow time and residence time in thin-film evaporators has been attempted through modifications of the rotary blades in the evaporators, such as, for example, by the use of a tapered thin-film evaporator as set forth in U.S. Pat. No. 2,927,634 or by employing wedges on rotary blades as set forth in U.S. Pat. No. 3,348,600.
In particular, U.S. Pat. No. 3,357,477, issued Dec. 12, 1967, describes a rotary thin-film-type liquid evaporator which employs helical rotor blades to control the flow path and residence time of a material being processed through the evaporator. In one embodiment, the rotor blades are twisted throughout their entire length in order to control the flow and residence time. As set forth in the patent, the degree of offset or the extent of the helical twist of the blades employed is critical, since the degree of offset cannot be so large that the vector forces developed prevent the processing of material or cause process material to remain in the evaporator for too long a time period, resulting in thermo-degradation of the material. In this patent, rotor blades with helical turns of 180.degree. or more have been found to be unsatisfactory for control purposes, since the back force created by such large extent of blade offset is so great that it becomes extremely difficult or impossible to process material through the evaporator. Typically then where the blade is helically twisted over the entire length, the extent of the helical twist is not more than about 1/2 to 30.degree. of twist per linear foot of blade length. This patent does not discuss and is not directed to the problems associated with the diminution of the film thickness as the film decreases in thickness from the feed to the product end on the interior wall of the evaporator.